The invention relates to a device for controlling a speed of the mandrel in continuous retained-mandrel rolling mills.
It is known that when a perforated bloom secured to a mandrel is continuously rolled, the rolls in the various housings subject the mandrel to tensile forces which, if the mandrel was loosely mounted, would drive it at a speed which varied during the rolling operation. It is also known that variations in the speed of the mandrel through the successive housings will result in unacceptable defects in the final rolled tube.
Accordingly, to avoid this serious disadvantage, the mandrel must be moved at a controlled speed.
To this end continuous rolling mills used for the aforementioned method of rolling are fitted with devices which oppose the tensile forces exerted by the rolls on the mandrel and move it at a predetermined speed through the successive housing. In the prior art, these devices are usually called "mandrel-retaining devices", whereas a rolling mill fitted with such devices is called a "continuous retained-mandrel rolling mill".
The retaining devices used hitherto are mainly of the screw or hydraulic kind.
In the prior art screw devices, one end of the mandrel is suitably shaped (i.e. the mandrel head) and engages a corresponding seat formed centrally in a cross-member, the ends of which are formed with two female threads each coupled to a screw having a length adapted to retain the mandrel so that it moves over a required predetermined distance.
In the hydraulic devices, the cross-member bearing the mandrel head is laterally secured to oil actuated cylinders which have the stroke required for retaining the mandrel so that it moves in the desired manner.
It is particularly difficult to control the speed of a mandrel since, during lamination, the tensile forces exerted on the mandrel may vary suddenly as the mandrel is engaged by the successive housings. Furthermore the tensile forces, particularly in large rolling mills, may reach peaks of 400 tons, so that the mandrel moves at speeds of up to 1 m/second. Consequently, screw retaining devices give satisfactory results only at moderate speeds and moderate, constant loads. However, screw retaining devices are quite inadequate for high speeds or loads, since the specific pressures on the sliding surfaces, the speed of sliding and the difficulty of lubrication result in rapid deterioration of the screw and thread couplings.
In hydraulic retaining devices, the heavy loads accompanied by sudden variations in intensity produce resonance effects owing to the compressibility of the hydraulic fluid and the resilience of the tubes, thus causing the mandrels to vibrate sufficiently to break the lubricating layer interposed between the mandrel and the rolled bloom. As is well known, a break in the lubrication adversely affects the life of the mandrels, resulting in variations in the tensile force exerted on the mandrel, and resulting in further resonance in the hydraulic retaining device, with a cumulative negative effect on the entire rolling operation.
The invention is based on the problem of devising a mandrel-retaining device in continuous retained-mandrel rolling mills. The device includes structural and functional characteristics which completely obviate the aforementioned disadvantages of the prior art.
To accomplish the result, according to the invention, the device comprises:
a rod having a double rack provided at one end with a means for removably securing an end of a mandrel,
a number of pinions engaging the rack on the rod, each pinion being driven in rotation by a corresponding motor-driven shaft which, via respective clutches is connected to:
a motor and speed-reducing group provided with a brake for each motor-driven shaft.
According to another feature of the invention, the device comprises a second set of pinions engaging the rack on the rod, each pinion being driven in rotation by a corresponding motor and speed-reducing group provided with a brake.